Our objective is to characterize the nature of a dehydrogenase-isomerase system of T. pyriformis by purification of the dehydrogenase and isolation of the pathway intermediates. Because the dehydrogenase is active with both steroids and non-steroids as substrates we will study the substrate specificity and kinetics of the partially purified enzyme. A growth inhibition study will also be done to investigate the hypothesis that these transformations are related to the toxicity of steroids to this protozoan.